The present invention relates to a drum brake for a vehicle, or the like, and more particularly, to an improvement in a duo-servo type drum brake.
A drum brake used in a vehicle, or the like, generally comprises an input section (a wheel cylinder); brake shoes which expand in a drum so as to generate braking force upon receipt of force from the input section; and an anchor section for supporting the brake shoes.
Several types of conventional drum brakes, such as leading-trailing type drum brakes, two-leading type drum brakes, duo-servo type drum brakes, or the like, are used in general.
The leading-trailing type drum brake comprises a pair of opposed shoes, a wheel cylinder disposed between the opposed edges at one longitudinal end of the pair of shoes for expanding the shoes; and an anchor section disposed between the opposed edges at the other longitudinal end of the pair of shoes. In short, the leading-trailing type drum brake has a leading shoe and a trailing shoe incorporated therein.
The two-leading type drum brake has a pair of leading shoes incorporated therein. Of the two-leading type drum brakes, a single-acting two-leading type drum brake (TPLW) provides a large gain at the time of forward movement of the vehicle. In contrast, at the time of rearward movement of the vehicle, two trailing shoes are used to provide a comparatively smaller gain in comparison with the gain provided at the time of the forward travel. A double-acting two-leading type drum brake (TP2W) produces an equal amount of braking effect and provides a large gain for both forward and rearward movements of the vehicle.
The duo-servo type drum brake comprises a pair of shoes linked together. The braking force produced by the primary shoe is applied to the secondary shoe as input force, thereby effecting an equal amount of braking effect for both forward and rearward movements and resulting in a large gain.
However, the two-leading type drum brake requires two wheel cylinders, thereby rendering the drum brake expensive and a mechanism of a parking brake installed therein complicated.
In contrast, the duo-servo type drum brake has many advantages in comparison with the leading-trailing type drum brake or the two-leading type drum brake; for example, the advantages of considerably large braking effect, ease-of-miniaturization, and easy incorporation of a parking brake. On the other hand, for example, the duo-servo type drum brake is very sensitive to the frictional coefficient of a lining of the brake shoe or variations in the state of contact between the brake shoe and a rotary drum, thereby resulting in great variations in the braking effect and hence unstable braking characteristics.
In contrast, the leading-trailing type drum brake has superior stability and allows easy incorporation of a parking brake in comparison with the other two brakes. For this reason, in recent years, the leading-trailing type drum brake has become predominant.
The leading-trailing type drum brake produces a small amount of braking effect. To compensate for the deficiency of braking effect, a servo mechanism of the master cylinder or the diameter of the drum must be increased, which in turn makes it difficult to render the drum brake compact.